1. Field of the Invention
The present invention relates to a process for manufacturing a granulated igniter used in a gas generator container. The gas generator container may be incorporated, for example, into an automotive air bag unit. The granular igniter functions to quickly and uniformly ignite a solid gas generator contained in the gas generator container.
2. Description of the Related Art
Boron niter as a boron and potassium nitrate compound is an igniter containing boron and potassium nitrate as major ingredients. Boron niter has excellent heat stability, burns quickly and generates a high calorific value. Another desirable characteristic of boron niter is its relatively stable burn rate in the presence of ambient pressure fluctuations. Due to these characteristics, boron niter has been used as an igniter for rocket propellants, and recently, as a constituent of a gas generator containers to inflate air bags. Most recently, the consumption of boron niter has drastically increased due to wide spread use of automobile air bags.
Because boron niter can be ignited by impact or friction, it has up to now been produced in a small amounts, e.g., of about 0.5 to 20 kg/batch, in order to prevent unintended ignition.
The following is a common method of manufacturing boron niter. First, in a mixing step, powdery raw materials such as boron, potassium nitrate, etc. are mixed, a binder component dissolved in an organic solvent is added to the mixture, and the entire mixture is then subjected to wet blending. The blended mixture is next granulated in a granulation step by passing it in wet form through a wire or silk netting. The granule obtained is then dried to evaporate the solvent, and in a final step, in which it is filtered, or as it is known, classified.
The prior art method described above, however, involves the following five problems.
First, the granulation step, according to previous manufacturing methods, is a step which must be carried out very carefully. If large amounts of the granule are granulated at the same time, the blended mixture may inadvertently explode. Moreover, previous methods of manufacturing boron niter often entailed an inordinate number of steps from mixing of raw materials to classification. As a result, boron niter production required the use of large-scale equipment to achieve full remote control of these steps. Thus, a tremendous investment had to made in the equipment necessary for boron niter's production. Moreover, a lot of labor was required for the maintenance and control of the equipment. Were the granule to be prepared in the absence of such large-scale equipment, workers would be forced to directly participate in the manufacturing operation. Due to the igniter's explosiveness, countermeasures would therefore have to be taken to ensure the safety of the workers.
Secondly, about 10 to 20% by weight of micropowder is typically formed during the classification step according to previous manufacturing methods. Since the fluidity of the final product is inhibited by the presence of micropowder, the micropowder must be removed by all means. Accordingly, the yield of the final product will be lowered.
Thirdly, about 1 to 10% by weight of an organic binder is required by previous manufacturing methods to improve granulating properties in the granulation step. When burned, however, the organic binder produces toxic gasses such as carbon monoxide and hydrogen fluoride. Thus, if such organic binder is incorporated into a gas generator container for an air bag, the driver and passengers in the cabin of an automobile would be subject to inhaling toxic gasses.
Fourthly, since the shape of the granules produced by previous manufacturing methods lack a spherical shape, the granules consequently lack fluidity. Accordingly, the gas generator containers and the like are manufactured with poor efficiency. Moreover, the apparent specific gravity of the granule significantly varies from lot to lot. Thus, if a fixed weight of boron niter is to be loaded in a gas generator container, the volume with which it occupies inevitably varies, forcing the volume of the boron niter to be adjusted lot by lot. This makes for an intricate and difficult assembly of the gas generator containers.
Finally, according to previous manufacturing methods, the large number of steps needed to produce the igniter results in increased igniter manufacturing costs.